1. Field of the Invention
This invention pertains to holddown devices, and more particularly to apparatus for retaining flexible membranes to flat surfaces, a method of manufacturing the holddown apparatus or device, a roofing system incorporating the holddown device, and a method of constructing the roofing system.
2. Description of the Prior Art
It is well known to cover the exterior surface of a roof with sheets or membranes of flexible waterproof material. To retain the membranes in place on the roof, various types of disc-like stress or seam plates have been developed. For example, FIG. 1 shows a simplified view of a prior stamped metal seam plate 1 in use to retain a membrane 3 upon a typical roof deck 5. A layer of insulation 13 is normally placed between the deck 5 and the membrane 3. The seam plate 1 has a circular periphery 7 and a bottom surface 9. When the seam plate is stamped, a burr is often formed along the circular edge 15 between the periphery 7 and the bottom surface 9. The seam plate is clamped to the roof deck with the seam plate bottom surface 9 placed against the membrane 3. The seam plate is mechanically attached to the roof deck by a screw 11 that passes through a central hole of the seam plate and engages the roof deck.
When a wind storm occurs, the wind acts to pull the roof from overhead. This phenomena is known as negative uplift pressure or pullover pressure, because the resulting force is away from the building roof. The higher the wind speed, the greater the pullover pressure. As a result, substantial uplift pressure is often produced on the membrane 3, and the membrane is pulled toward the sky. In high winds, the upward force on the membrane creates a large force on the seam plate 1 as well as on the screw 11. In some instances, the seam plate may even bend slightly.
FIG. 2 shows in simplified fashion a membrane 3 that is under strong uplift pressure. The membrane becomes creased along the edge 15 between the periphery 7 and the bottom surface 9 of the seam plate 1. During excessively high winds, the burr around the seam plate edge 15 tends to tear the membrane, thus possibly initiating membrane failure.
Other examples of prior seam plates for roofing membranes may be seen in U.S. Pat. Nos. 4,890,968; 4,945,699; and 5,018,329. The seam plate of U.S. Pat. No. 4,945,699 includes prongs that grip the membrane. Careful attention must be paid to the design and manufacture of that seam plate to assure that the prongs do not puncture through the membrane.
Another prior seam plate is made from molded plastic and has a dozen or more rather long needle-like prongs. That seam plate is undesirably expensive, and the prongs have a tendency to weaken the membrane.
Thus, a need exists for improvements in the seam plates that retain a flexible membrane to a roof.